The present invention concerns the improved process for preparing chlorinated pyridines from alpha-picoline. The present invention particularly concerns the preparation of monochloropyridine, 2,6-dichloropyridine, 2,3,6-trichloropyridine, and 2,3,5,6-tetrachloropyridine.
The chlorinated pyridine derivatives of the present invention are known compounds having been previously prepared by a number of processes. These compounds, namely 2,3,5,6-tetrachloropyridine, have uses such as herbicides, pesticides, etc., and are also employed as chemical intermediates in the preparation of other highly desired herbicide or pesticide products. Previous methods for preparing such compounds include those described in the following patents as well as the prior art noted therein: U.S. Pat. Nos. 3,186,944; 3,370,062; 3,420,833; 3,538,100; 3,732,230; 3,969,205; European Pat. No. 9,212; Great Britain Pat. No. 1,532,038; and Japanese patent publication No. 75/154,266.
Thus, U.S. Pat. No. 3,969,205 describes the photochemical reaction of pyridine and chlorine to prepare 2-chloropyridine. Japanese patent publication No. 75/154,266 describes a similar reaction between chlorine and pyridine or 2-chloropyridine to prepare 2,6-dichloropyridine.
GB No. 1,532,038 describes the preparation of 2,3,5-trichloropyridine by the gas phase reaction of 3,5-dichloropyridine with a large excess of chlorine at temperature of 300.degree.-460.degree. C.
Mixtures of trichloro-, tetrachloro-, and pentachloropyridine are reported by U.S. Pat. No. 3,186,994 to be formed by the chlorination of a polychloro- (trichloromethyl)-pyridine in the liquid phase using either irradiation with ultraviolet light or temperatures above 400.degree. C.
U.S. Pat. No. 3,538,100 describes the preparation of 2,3,5,6-tetrachloropyridine and pentachloropyridine by the chlorination of liquid 2,6-dichloropyridine at a temperature of at least 180.degree. C. in the presence of a metallic halide catalyst.
Picolines have been reported to react with chlorine under a variety of conditions to form mixtures of chloropicolines. Thus, EP No. 9,212 reports using .beta.-picoline in the gas phase in the presence of silica or alumina to form 2-chloro-5-trichloromethylpyridine. U.S. Pat. No. 3,420,833 reports the reaction of .alpha.-picoline using carbon tetrachloride or water as a diluent to form a mixture of chlorinated picolines. U.S. Pat. No. 3,732,230 reports the liquid phase chlorination of alpha-picoline hydrochloride containing no more than 5 percent of water. Here too the result was a mixture of chlorinated picolines. In neither of these references listed above involving the chlorination of picoline is there a report of the cleavage of the alkyl group to form chlorinated pyridines. The alkyl group is always preferentially chlorinated; and the resulting trichloromethyl group retained throughout the reaction.
U.S. Pat. No. 3,370,062 reported the reaction of .alpha.-picoline with chlorine in the presence of silica or alumina catalysts to primarily form pentachloropyridine. However this process requires the use of a fluidized bed and only affords yields of less than 50 percent. In addition, this reaction proceeded directly to pentachloropyridine, without any ability to obtain the highly desired lower chlorinated pyridines, such as 2,3,5,6-tetrachloropyridine.